By applying voltage to an organic electroluminescence device (also referred to as “organic EL device”), holes from an anode and electrons from a cathode are injected into a light emitting layer. The holes and electrons injected into the light emitting layer recombine to form excitons. The singlet exciton and the triplet exciton are formed at a ratio of 25%:75% according to spin-statistics theorem. Since the fluorescence utilizes the emission from singlet excitons, it has been known that the internal quantum efficiency of a fluorescent organic EL device is limited to 25%. In contrast, since the phosphorescence utilizes the emission from triplet excitons, it has been known that the internal quantum efficiency of a phosphorescent organic EL device can be increased to 100% if the intersystem crossing occurs efficiently.
In the development of known organic EL devices, an optimum device design has been made depending upon the emission mechanism such as fluorescence and phosphorescence. It has been known in the art that a high-performance phosphorescent organic EL device cannot be obtained by a mere application of the fluorescent technique to the phosphorescent device, because the emission mechanisms are different from each other. This may be generally because the following reasons.
Since the phosphorescence utilizes the emission from triplet excitons, a compound with larger energy gap is required to be used in the light emitting layer. This is because that the singlet energy (energy difference between the lowest excited singlet state and the ground state) of a compound is generally larger than its triplet energy (energy difference between the lowest excited triplet state and the ground state).
Therefore, to effectively confine the triplet energy of a phosphorescent dopant material within a device, a host material having triplet energy larger than that of the phosphorescent dopant material should be used in the light emitting layer. In addition, if an electron transporting layer and a hole transporting layer is formed adjacent to the light emitting layer, a compound having triplet energy larger than that of the phosphorescent dopant material should be used also in the electron transporting layer and the hole transporting layer. Thus, the device design conventionally employed for developing a phosphorescent organic EL device results in the use of a compound having an energy gap larger than that of a compound for use in a fluorescent organic EL device, thereby increasing the voltage for driving an organic EL device.
A hydrocarbon compound highly resistant to oxidation and reduction, which has been known as a useful compound for a fluorescent device, has a small energy gap because of a broad distribution of n-electron cloud. Therefore, such a hydrocarbon compound is not suitable for use in a phosphorescent organic EL device and, instead, an organic compound having a heteroatom, such as oxygen and nitrogen, has been selected. However, a phosphorescent organic EL device employing such an organic compound having a heteroatom has a shorter lifetime as compared with a fluorescent organic EL device.
In addition, a phosphorescent dopant material has an extremely longer relaxation time of triplet excitons as compared with that of its singlet excitons, this largely affecting the device performance. Namely, in the emission from singlet excitons, since the relaxation speed which leads to emission is high, the diffusion of excitons into a layer adjacent to the light emitting layer (for example, a hole transporting layer and an electron transporting layer) is difficult to occur and efficient emission is expected. In contrast, the emission from triplet excitons is a spin-forbidden transition and the relaxation speed is low. Therefore, the diffusion of excitons into adjacent layers occurs easily and the thermal energy deactivation occurs in most compounds other than the specific phosphorescent compound. Thus, as compared with a fluorescent organic EL device, it is more important for a phosphorescent organic EL device to control the region for recombining electrons and holes.
For the above reasons, the development of a high performance phosphorescent organic EL device requires the selection of materials and the consideration of device design which are different from those for a fluorescent organic EL device.
A carbazole derivative having a high triplet energy and a carbazole skeleton known as a principal skeleton of hole transporting materials has been conventionally used as a useful phosphorescent host material.
Patent Document 1 describes, as a material for organic EL device, a compound in which a nitrogen-containing heterocyclic group is introduced into a biscarbazole skeleton which includes two carbazole structures connected to each other. The compound described in Patent Document 1 is molecularly designed to balance the charge transport by introducing an electron-deficient nitrogen-containing heterocyclic group to a hole transporting carbazole skeleton. Patent Document 2 describes that the charge injecting ability of a N,N-biscarbazole compound wherein two carbazole structures are bonded to each other via a biphenyl group is improved by introducing an electron-withdrawing group into the intervening biphenyl group between two carbazole structures.
However, the improvement of the lifetime of organic EL device is still required and the development of a new material for organic EL device which realizes a longer lifetime has been demanded.